1. Field of the Invention
The present invention relates to liquid immersion exposure technology used in a process of manufacturing a device, such as a semiconductor device.
2. Description of the Related Art
A reduction projection exposure apparatus is used in manufacturing a semiconductor device (e.g., a large-scale integrated (LSI) circuit or very-large-scale integrated (VLSI) circuit) having an ultra-fine pattern. The reduction projection exposure apparatus projects, in a reduced size, a pattern formed on a mask onto a substrate coated with a photosensitive agent and transfers the pattern to the substrate. As the degree of integration in semiconductor devices increases, it becomes necessary to produce finer circuit patterns. With the development of resist processes, efforts have been made to develop exposure apparatuses that can achieve finer patterning.
A typical method for increasing the resolution of an exposure apparatus is to shorten the exposure wavelength or to increase the numerical aperture (NA) of a projection optical system. As for the exposure wavelength, there is an ongoing shift from the use of a 365-nm i-line to the use of light emitted from a KrF excimer laser having an oscillation wavelength of about 248 nm. Additionally, an ArF excimer laser having an oscillation wavelength of about 193 nm is under development. A fluorine (F2) excimer laser having an oscillation wavelength of about 157 nm is also under development.
At the same time, a projection exposure method using a liquid immersion method is now gaining attention. The liquid immersion method is also a technique for increasing resolution, but is completely different from the other methods described above. In conventional methods, a space between the last surface of a projection optical system and a substrate surface (e.g., wafer surface) to be exposed is filled with a gas. However, in the liquid immersion method, projection exposure is performed while this space is filled with a liquid. An advantage of the liquid immersion method is that if the refractive index of a liquid filling the space between a projection optical system and a wafer is “n”, it is possible to achieve a resolution that is “n” times higher than that achievable by conventional methods which uses light sources having the same wavelength as that of a light source used in the liquid immersion method. When, for example, the space between the projection optical system and the wafer is filled with pure water (having a refractive index of 1.33), if the maximum angle of incidence of a light beam forming an image on the wafer in the liquid immersion method is the same as that in the conventional methods, a resolution achieved by the liquid immersion method is 1.33 times higher than that achieved by the conventional methods even if the wavelength of the light source used in the liquid immersion method is the same as that of the light sources used in the conventional methods. This is equivalent to increasing the NA of the projection optical system in the conventional methods to 1.33 times. The liquid immersion method can achieve a resolution corresponding to an NA of 1 or more, which is not achievable by the conventional methods.
Examples of conventional exposure apparatuses to which the liquid immersion method is applied include an exposure apparatus disclosed in Japanese Patent Laid-Open No. 6-124873. FIG. 9A and FIG. 9B illustrate a structure of this exposure apparatus. In the exposure apparatus of FIG. 9A, filling a liquid tank (chamber) 109 with a liquid 130 allows the space between the last surface (optical element 107) of a projection optical system 104 and a wafer 102 to be filled with the liquid 130. In the liquid tank 109, there are disposed all or part of coarse positioning units 111-1 to 111-4 as well as a wafer conveying unit for conveying the wafer 102 from a wafer cassette 110 and placing the wafer 102 on a wafer chuck 112. The wafer chuck 112, an XY stage 113, and a fine-motion stage 114 are also disposed in the liquid tank 109. In FIG. 9A, a reference mirror 116 is attached in the X and Y directions (Y direction is not shown) to the fine-motion stage 114 and reflects light from a laser interferometer 115 to measure the position of the fine moving stage 114. The liquid tank 109 has a window 117 which allows light from the laser interferometer 115 to pass therethrough. A heat insulator 118 outside the liquid tank 109 thermally insulates the liquid tank 109 from outside.
Upon completion of exposure of the entire surface of the wafer 102 to light in the exposure apparatus of FIG. 9A, a conveying pump 122 is activated again and starts discharging the liquid 130 from the liquid tank 109. A liquid level gauge 119, which constantly monitors the level of the liquid 130, causes the conveying pump 122 to stop when the level of the liquid 130 becomes slightly lower than that of the surface of the wafer chuck 112. Therefore, the amount of the liquid 130 discharged from the liquid tank 109 is small. After that, a vacuum of the wafer chuck 112 is turned off. Then, the wafer 102 on the wafer chuck 112 is moved with a conveying hand 111-4 and stored in the wafer cassette 110. At a point immediately before the storage of the wafer 102 in the wafer cassette 110, the liquid 130 may be removed from the wafer 102 by blowing clean air onto both surfaces of the wafer 102.
As illustrated in FIG. 9B, Japanese Patent Laid-Open No. 6-124873 also discloses a structure in which only part including the wafer chuck 112 is disposed in the liquid tank 109. A structure in which the wafer chuck 112 is directly disposed on the bottom surface of the liquid tank 109, which is disposed on the fine-motion stage 114, is also disclosed. That is, Japanese Patent Laid-Open No. 6-124873 disposes the exposure apparatus in which the last surface of the projection optical system 104 and the entire wafer 102 are disposed in the liquid tank 109.
International Patent Publication No. WO99/049504 describes another example in which the liquid immersion method is applied to an exposure apparatus. International Patent Publication WO99/049504 discloses an exposure apparatus in which a liquid is supplied exclusively to the space between a projection optical system and a wafer surface such that the space is filled with the liquid.
FIG. 10 illustrates an inline connection between an exposure apparatus 153 and a coating/developing device 152 disclosed in Japanese Patent Laid-Open No. 2002-057100. Upon receipt of a wafer from the coating/developing device 152, a wafer conveying robot of the exposure apparatus 153 conveys the wafer to a wafer stage (exposure stage) 155. The same wafer conveying robot also conveys the exposed wafer to the coating/developing device 152. In the coating/developing device 152, a robot which conveys a wafer coated with a resist to an interface 154 adjacent to the exposure apparatus 153 conveys the exposed wafer to a heating unit.
In the conventional exposure apparatuses described above, even if a liquid between the projection optical system and the wafer surface is discharged to the outside after exposure, some liquid may remain on the last surface of a projection lens. This means that the liquid adhering to the last surface of the projection lens may be dropped onto the wafer during handling of the wafer. Moreover, the liquid on the wafer may further be dropped onto the wafer chuck during conveyance of the wafer, cause local defocusing, and lead to lower yields. Additionally, this requires replacement of the wafer chuck, involves a considerable amount of apparatus downtime associated with the replacement, and thus significantly affects the device productivity. Moreover, if the liquid adheres to the conveying hand during conveyance of an exposed and wet wafer, the liquid further adheres to an unexposed wafer to be subsequently conveyed. In other words, cross-contamination occurs. This causes local defocusing and leads to lower yields.
Furthermore, if a wafer to which a liquid adheres is conveyed to the coating/developing device and subjected to heat treatment, heat of evaporation of the liquid causes temperature non-uniformity and degradation in critical dimension (CD) uniformity. If high-speed spin drying or the like is performed under assumption that all exposed wafers are wet, the structure of the exposure apparatus is made complex. This causes an increase in cost and a reduction in throughput.